I What is the minimum force required to lift an object?

[Herman Trivilino]

A person stands on a scale. The scale reads his mass 60 kg . Now this human moves up his body short distance like someone tries to pick a fruit from a tree. The scale will start to increase by small forces x N in which the total read of the scale is 600+x N *. The force he exerts on the scale is x N. The force the scale pushes him up is also x N two forces in opposite directions. The force that lifts his body is the force the scale pushes him up which turned out to be the x N.

This passage is confusing to me. When he moves up his body he will no longer be in contact with the scale and it will read zero. If, on the other hand he grabs a branch of the tree and pulls upward while still in contact with the scale, the reading on the scale will decrease. If instead he pushes upward on the tree branch the reading on the scale will increase.

Let's say the scale reads Newtons. If he pushes upward on the tree branch with a force of x, then the reading on the scale will be 600 N + x.

You can confirm all this for yourself. Place a bathroom scale on the floor next to a table. Stand on the scale. If you push down on the table with your hand the reading on the scale will decrease.

 

[Herman Trivilino]

If there's a 100N weight on top of the table, how many N is it going to require to lift it vertically off the table ?

100 N. Once you apply the force of 100 N in the upward direction, the force exerted on the table top by the object you're lifting is zero. The net force exerted on the object is zero (100 N weight force downward and 100 N force pulling upward). Thus the object will either be at rest or move with a steady speed.

You can try this for yourself. Use a scale that you hang things from to weigh them. Set an object on the table top, attach the scale to it, and lift by pulling upward on the scale. You will find that the minimum force needed to lift it is equal to the object's weight.

 

[Yahya Sharif]

This passage is confusing to me. When he moves up his body he will no longer be in contact with the scale and it will read zero. If, on the other hand he grabs a branch of the tree and pulls upward while still in contact with the scale, the reading on the scale will decrease. If instead he pushes upward on the tree branch the reading on the scale will increase.

Let's say the scale reads Newtons. If he pushes upward on the tree branch with a force of x, then the reading on the scale will be 600 N + x.

You can confirm all this for yourself. Place a bathroom scale on the floor next to a table. Stand on the scale. If you push down on the table with your hand the reading on the scale will decrease.

He moves his body up short distance with his feet and calves' muscles* while his toes touch the scale the heel moves up.
* by the way the calves' muscles are weak so they provide with this small x N and can't provide a 600 N force. I tried once to measure my calves' muscles force by lying and pressing a rope of a scale. My maximum calves' muscles force was 80 N. And of course I lift my body comfortably far lesser than 80 N.

 

[PeroK]

How a person of 60 kg can jump high and fast against gravity with his only leg muscles while he will barely move a rock of 60 kg with his all body muscles? He uses small force x N to lift his body and a small force to jump.

When I used to go to the gym I could pull down 60kg with my arms and shoulders. I could repeat that up to12 times.

It might be awkward to move a large stone because of its shape, but if you put it on a carriage on wheels or rails, then it's easily movable with a small force.

To be honest, you are not really using physics here in this thread, but a manic misrepresentation of physics. Instesd of insisting that your alternative theories are correct, you should stay calm and learn!

 

[russ_watters]

The difference is big. What force I need to throw a rock of 60 kg in the air? what force of my legs I need to throw my body 60 kg jumping?

Throwing is a lot different from lifting and carrying. To lift and carry you are carrying the rock plus your weight. Throwing requires significant additional force to accelerate the rock. Our bodies are not made for accelerating large objects like that: our arms are much weaker than our legs. [edit] corrected.

The force I need to throw the rock is hundreds times the force I need to jump even though the body and the rock are of the same mass.

Hundreds of times? Nonsense. The forces applied two two 60 kg objects to get the same result must be exactly the same. It's just (again) our bodies aren't built for throwing 60 kg rocks. But if you lie on your back and can shove the rock with your legs, you might get a similar result to jumping (sending it maybe half a meter to a meter in the air). This force might be several times your weight.

You are frequently changing scenarios here, and I think you are confusing yourself. You should focus on fully understanding one well defined scenario at a time. Unless this is all a tactic for being argumentative...

 

[Yahya Sharif]

Our bodies are not made for accelerating large objects like that: our legs are much weaker than our arms.

How I accelerate a 60 kg body against gravity that high when jumping?I can do jumping with arms upside down and with legs.

 

[malawi_glenn]

I can do jumping with arms upside down and with legs.

Pics or it didn't happen!

What force I need to throw a rock

You have to train your upper-body muscles too you know, don't be that guy who lives in the squat rack! Give those pecs, delts and triceps some love! Often neglected when it comes to building a physique which can throw 60 kg stones like the newspaper delivery boy does with his paper dumbells - core and forearm strength! The plank exercise is good, try to use a friend if you have any (I don't) who can sit on your back while performing it.

our legs are much weaker than our arms.

Stop skipping legday, you are supposed to squat in the squat-rack not performing biceps curls

 

[russ_watters]

How I accelerate a 60 kg body against gravity that high when jumping?I can do jumping with arms upside down and with legs.

FYI, I said that backwards. Obviously its our legs that are much stronger than our arms. Anyway, you can easily do the math on this if you want. It's a fairly simple problem to calculate the force over distance required to jump a certain height. We've done it recently, modeling the legs as a spring (but personally I think it's probably more accurate as a constant force over distance). And if you have ever done or watched weightlifting, you'll know that people routinely lift 2-5 times their weight (including their body itself) with their legs. That's the range of force we're talking about.

 

[Herman Trivilino]

He moves his body up short distance with his feet and calves' muscles* while his toes touch the scale the heel moves up.
* by the way the calves' muscles are weak so they provide with this small x N and can't provide a 600 N force.

If his feet are still in contact then all of my post, except for the first two sentences, are still valid. My points are that you are making a sign error as well as other errors.

 

[Yahya Sharif]

Sorry I meant in my #36 post that I disagree with russ_watters, he says humans cannot accelerate large objects like rocks but I see that I can accelerate my large body very high by jumping using only leg muscles.

 

[russ_watters]

Sorry I meant in my #36 post that I disagree with russ_watters, he says humans cannot accelerate large objects like rocks...

I said no such thing. What are we doing here/what is the point of all this argumentation?

 

[Herman Trivilino]

How I accelerate a 60 kg body against gravity that high when jumping?

Hold on tight to the 60-kg body when you jump. If you have a mass of 60 kg then when you jump you are accelerating a system whose mass is 120 kg. The system thus weighs 1200 N. If you exert a force upward whose magnitude is greater than 1200 N, the body will accelerate upward (against gravity, as you say).

This happens, for example, when a new spouse is carried across a threshold that requires a step up, assuming both you and your spouse each have a mass of 60 kg.

Note that you can cheat a bit on this by lowering the body or parts of your body a bit during the jump, which is why I said "hold on tight". We are essentially assuming our system is a particle.

 

[hmmm27]

100 N. Once you apply the force of 100 N in the upward direction, the force exerted on the table top by the object you're lifting is zero. The net force exerted on the object is zero (100 N weight force downward and 100 N force pulling upward). Thus the object will either be at rest or move with a steady speed.

You can try this for yourself. Use a scale that you hang things from to weigh them. Set an object on the table top, attach the scale to it, and lift by pulling upward on the scale. You will find that the minimum force needed to lift it is equal to the object's weight.

That was a question for the OP, but thanks for caring

 

[Yahya Sharif]

That was a question for the OP

I disagree. Let's say I applied a 50 N on the object it will not move and while I increase the force it will still remain. When I reach the 100 N it will still remain because the net force I apply is zero So I need to go beyond the 100 N by any force greater than 100 N to lift the object. The force is actually the smallest number greater than 100 N which is actually cannot be determined so when using the scale you cannot know whether the force is 100 N or greater because any force like 100.000001 N can cause non-zero net force and lift the object.

 

[PeroK]

Observations:

To lift a rigid object of mass $m$ off the ground, you must apply a force greater than $mg$.

A rigid object cannot jump.

A human body with a muscle system that can release energy can jump. Although not from a standing position. One must bend the knees.

From the energy perspective the released energy must go to kinetic energy of the body, given the constraint of initially being in contact with the ground.

The muscles act so as to maintain a force against the ground even when the body Is accelerating upwards. That is the critical biomechanical difference between a human and a rigid body: the legs are still pushing on the ground during acceleration.

 

[hmmm27]

If the force exerted by the feet, flat on the ground is 600N, then

to raise the body upwards - to stand on the toes - requires the ankles to supply a vertical force of >600N.

 

[jbriggs444]

I can accelerate my large body very high by jumping using only leg muscles.

"Very high"? Typically less than 18 inches measured between standing and peak-of-arc centers of gravity.

Jumping over obstacles higher than this is possible because one's center of gravity starts two or three feet off of the floor, does not need to pass over the high jump bar and because high jumpers are more athletic than an average person.

Hurling a 60 kg rock to a height of 18 inches above my outstretched fingertips after having first picked it up from the ground and then having hoisted it over my head sounds like a rather more difficult task than jumping 18 inches.

 

[Yahya Sharif]

"Very high"? Typically less than 18 inches measured between standing and peak-of-arc centers of gravity.

Sorry for my English. I meant very high amount of acceleration. I can somehow measure it by hitting a roof above my head which will give immense strike.

 

[jbriggs444]

Sorry for my English. I meant very high amount of acceleration. I can somehow measure it by hitting a roof above my head which will give immense strike.

Typically less than one gee of vertical acceleration -- the distance that one crouches to make a jump is comparable to the height achieved as a result.

Measuring launch acceleration based on the pain endured in the resulting head strike is not widely regarded as an accurate technique.

By contrast, I would regard measurement based on the height of the ceiling above the height of the top of one's head as an accurate, albeit cumbersome technique. [Repeated jumps while slowly raising the ceiling until the head stops bonking].

 

[Ibix]

By contrast, I would regard measurement based on the height of the ceiling above the height of the top of one's head as an accurate, albeit cumbersome technique. [Repeated jumps while slowly raising the ceiling until the head stops bonking].

A rare case where beating someone over the head with something might actually advance their understanding. Well played, sir!

 

[Herman Trivilino]

When I reach the 100 N it will still remain because the net force I apply is zero So I need to go beyond the 100 N by any force greater than 100 N to lift the object.

When the object is moving upward at a steady speed the force you apply is 100 N. Not one bit more. A bit more would cause it to accelerate.

 

[Jodo]

I disagree. Let's say I applied a 50 N on the object it will not move and while I increase the force it will still remain. When I reach the 100 N it will still remain because the net force I apply is zero So I need to go beyond the 100 N by any force greater than 100 N to lift the object. The force is actually the smallest number greater than 100 N which is actually cannot be determined so when using the scale you cannot know whether the force is 100 N or greater because any force like 100.000001 N can cause non-zero net force and lift the object.

So a more accurate scale would prove this? One might wonder if 100.000001 N were needed to lift 100 N , would 100.00000997 work?
Something seems amiss here.

 

[Orodruin]

unless it is legday, then everything is allowed

What if every day is legday?

The force I need to throw the rock is hundreds times the force I need to jump even though the body and the rock are of the same mass.

No, it is not. Also keep in mind that it is not only a question of force but also of leverages. Your body is the result of millions of years of evolution and your legs shaped in such a way as to let you lift your body and move as efficiently as possible. It is less evolved to have the correct leverages to throw heavy objects. Lifting technique does help a lot but stones are also far less adapted to allow good lifting positions than say a barbell.

Back before Covid and becoming a father I used to powerlift for some time. I also helped my mother move some rocks at her summer house and I can tell you they seem much heavier for the same weight compared to a barbell simply because of worse leverages.

When your legs lift your body then that is all your body needs to do. If you also want to throw a heavy object then not only do your legs need to provide the force for both your weight and the object’s. The rest of your body, which as it has already been pointed out is significantly weaker, also needs to engage with typically bad leverage.

But if you lie on your back and can shove the rock with your legs, you might get a similar result to jumping (sending it maybe half a meter to a meter in the air).

Interestingly, there are gym machines designed specifically to do this in a more controlled manner:

I do not recommend loading it with so little that you can actually launch the foot plate in the air…

 

[malawi_glenn]

What if every day is legday?

Then you will never be able to throw that 60 kg stone and saying "ain't nothing put a peanut" whilst doing so

Interestingly, there are gym machines designed specifically to do this in a more controlled manner:

This is a quite more esotheric exercise, vertical legpress in a smith-machine

 

[Yahya Sharif]

This is a quite more esotheric exercise, vertical legpress in a smith-machine

This is an equivalent movement to jumping. The person is hardly lifting the load even though his muscles strength is above average. While he can jump without effort.

 

[malawi_glenn]

The person is hardly lifting the load even though his muscles strength is above average. While he can jump without effort.

What is your problem, really? What is that you do not understand about forces?
You want to know how much weight is put on that bar? Those were clean and easy reps, did not break a sweat!

You can also get muscle strength above average, just hit the gym bro instead of trying to deduce that the laws of physics does not apply to jumping or whatever you are trying to do here.

 

[Yahya Sharif]

When I try to lift my body as the experiment just before I lift my body the reading of the scale increases by the x force I mentioned in which the scale will read 60+x kgf when my body moves and accelerates upwards the x got a maximum of x2 N without accelerating upwards just my body at constant speed. What is this x? what its maximum value and why? The idea is for every body there is a minimum x force that can lift it. What is your opinions?

 

[russ_watters]

When I try to lift my body as the experiment just before I lift my body the reading of the scale increases by the x force I mentioned in which the scale will read 60+x kgf when my body moves and accelerates upwards the x got a maximum of x2 N without accelerating upwards just my body at constant speed. What is this x? what its maximum value and why? The idea is for every body there is a minimum x force that can lift it. What is your opinions?

I don't understand what you are asking. That's probably partly due to your long first sentence which reads like a run-on sentence and appears to contain multiple unconnected thoughts. And did you really mean to ask about minimum and maximum at the same time?

At this point though, you should understand this simple issue well enough to answer yourself - so why don't you tell us what you think the answers are? And maybe if you draw a diagram it will both help you understand the force balance and show us what you really mean by your scenario.

 

[Yahya Sharif]

In the video the weight is 1.29 kg. First I measured the weight then I started from zero Newtons to lift the weight the force continue to increase while I am trying to lift the weight. As soon as the weight raises the force reached its maximum 1.29 kgf in this case by definition the force I need to lift the 1.29 kg is 1.29 kgf which is equal to the weight.*

Now, in the case of the human body lifting his body, he starts pressing the scale, the x continues to increase, as soon as his body raises he reaches a maximum of x N then by definition the force to lift the body is the x N.

This x force is not a random force, it is a specific amount that is proportional to the human mass, If I repeat the lifting, as soon as my body raise I will get the same value if the human is a child of 25 kg the force in the scale to lift is smaller than x. So every human can lift his body with a specific force depending on his weight.

The x N force is the force that lifted the human body which is less than the weight 60 kg, in contrary of the physical fact that a force to lift a mass must be greater than the mass weight. This means a force smaller than the human body weight is sufficient to lift the body. What I mean is a human or an animal is an exception.
So what is this specific x N force ?*actually it is the smallest force greater than 1.29 kgf or 1.29+f the resultant is 1.29+f-1.29 or f, f is the force upwards I need this force to create a non-zero net force upwards. But as it is tiny it does not appear in the scale instead the scale will read 1.29 kg.

 

[jbriggs444]

The x N force is the force that lifted the human body which is less than the weight 60 kg, in contrary of the physical fact that a force to lift a mass must be greater than the mass weight.

Can you clarify, please.

You are saying that you exerted a force of less than 60 kg(force) and succeeded in lifting a human body with mass 60 kg(mass)? But you did not show us that experiment. Nor did you describe it.